Wire electrical discharge machining is a processing technique of generating electrical discharge (arc discharge) between an electrode wire for electrical discharge machining (hereinafter called an electrode wire) and a workpiece and cutting the workpiece using heat energy resulting from the electrical discharge. Wire electrical discharge machining is particularly suitable for processing metal having a complicated shape such as a mold.
The aforementioned wire electrical discharge machining is required to satisfy the following: (a) a high processing speed; (b) a favorable finished condition of a surface of the workpiece and favorable dimension accuracy of the workpiece; (c) favorable positioning performance during measurement of a position of the electrode wire and that of the workpiece relative to each other; (d) a small amount of metallic powder to be caused while the electrode wire is moved continuously; (e) favorable wire connection performance during placement of the electrode wire on an electrical discharge machine; and (f) low cost.
Various studies have been conducted on the structure of the electrode wire. There has been an electrode wire made of single brass, for example. A higher zinc concentration in the composition of the electrode wire is generally known to increase a processing speed. However, if a zinc concentration in the electrode wire made of single brass is 40 wt % or more, for example, an intermetallic product having a body-centered cubic lattice is formed. This reduces ductility and toughness to make it difficult to execute cold wire drawing process. In this regard, there has been a wide-used conventional electrode wire made of single brass having a zinc concentration from 35 to 40 wt %, for example.
To increase a processing speed further, an electrode wire including a zinc layer formed only on a surface of the wire has been suggested. The electrode wire coated with the zinc layer may contribute to increase in a processing speed. However, zinc having a low boiling point is evaporated instantaneously during wire electrical discharge machining. This turns out to limit a processing speed.
There has also been a composite electrode wire formed by coating a brass core with zinc and then forming a β-brass layer by thermal process (thermal diffusion process). The electrode wire with the β-brass layer is given enhanced antiwear performance during electrical discharge machining. However, a sufficient processing speed cannot be achieved.
In consideration of the aforementioned problems, conventional techniques disclosed in patent document 1 to 4 have been developed.
Patent document 1 suggests a structure where an electrode wire core is coated with a γ-brass layer. Formation of this electrode wire is as follows. The γ-brass layer is formed by thermal process on a zinc-coated base wire. Then, the base wire is drawn to form fractured γ-brass on a surface of the electrode wire.
Patent document 2 discloses an electrode wire where a surface layer of a core of the electrode wire has a multilayer structure. In the electrode wire described in patent document 2, the surface layer of the core has an inner layer mainly formed of a β-brass layer and an outer layer mainly formed of a γ-brass layer. As described above, the electrode wire suggested in patent document 2 includes both β-brass and γ-brass arranged in an external layer of the core of the electrode wire.
Patent document 3 discloses an electrode wire including a stack of the following formed in an external layer of a core material of the electrode wire: a β-brass sublayer, and fractured γ-brass that reveals β-brass in a fracture. Gamma-brass is superior to β-brass in terms of electrical discharge performance, and γ-brass is superior to β-brass in terms of evaporation of zinc. Thus, in this electrode wire, β-brass exists after evaporation of γ-brass. Further, a high processing speed is achieved.
Patent document 4 discloses an electrode wire where a surface of an external layer (made of β-brass, for example) of a core material of the electrode wire is given a structure with projections and recesses, and the recesses are filled with a filler made of evaporable metal (γ-brass, for example).